I have not transited BKK since they setup the new security arrangement. So do you have to show onward board pass to go through security and get back up to the departure levels?

Now, even if you can do it I dont think I would. There is not a lot to do besides have a meal and perhaps go to the spa for a massage. I would most likely go to get the hotel situations settled and perhaps get a shower if I was able to check-in early and come back to the airport and meet the parents. You could also possibly just get a day rate at the Novotel which is connected to the airport and get a shower etc and some rest while waiting for them. There are other properties near the airport now and they some have free shuttles so you may check those out also. I do a search on expedia and it will search by distance from the airport and that will show you the properties that are close.